The present invention relates to an improved method and apparatus for doping silicon and other crystals grown by the Czochralski process.
Single crystalline semiconductor materials such as silicon are widely used in the electronics industry, particularly as a starting material for integrated circuits. One method of forming single crystal silicon is known as the Czochralski process. In this process, a seed crystal, which has the desired crystal orientation, is introduced into a melt of a semiconductor such as silicon. The melt is contained in a quartz or graphite crucible or vessel which is heated so that the silicon melt is at or slightly above its melting point. Crystal growth is a very sensitive process. The slightest disturbance can alter the crystal and render it unsuitable for use in integrated circuits.
In the Czochralski process, the seed crystal is slowly withdrawn from the melt in an inert atmosphere such as argon as the crystal and melt are rotated such that the silicon solidifies on the seed to produce a larger cylindrical silicon crystal called a boule or ingot. The crucible is usually rotated in the opposite direction of the crystal rotation to assure thermal balance in the growth environment. The pull rate and the power are adjusted to cause the diameter of the crystal to increase in a cone shape until the desired crystal diameter is achieved. The withdrawal rate and heating are then adjusted to maintain the desired diameter until the crystal is terminated by increasing the power and pull rate to form a cone on the end of the crystal ingot.
Certain impurities or dopants are commonly introduced to the melt for the purpose of modifying the electrical characteristics such as the resistance of the crystal. Typical dopants for silicon semiconductor crystal include arsenic, boron, antimony and red phosphorus. One method of adding dopants is to drop pellets of the dopant material into the molten material. However, this method is very inefficient because the vaporizing temperature of volatile dopants such as arsenic is substantially lower than the temperature of the melt and, as a result, a substantial amount of the dopant is lost through evaporation. Another method that has been used to dope silicon with volatile dopants such as arsenic is to suspend a quartz basket containing the dopant above the melt in an bell jar. This method is also inefficient because a large portion of the vaporized arsenic tends to flow under and out of the bell jar and does not dissolve in the silicon melt.
Accordingly, there is a need for an improved method and apparatus for doping crystals with volatile dopants such as arsenic by the Czochralski process.